1. Field of the Invention
This invention relates generally to temperature-responsive protective circuits for electric bedcovers. It relates particularly to an electric bedcover of a type wherein a safety thermostat having a bimetallic element is adapted to deenergize a heater when the bimetallic element is heated sufficiently, as when an overheated condition obtains.
Adequate protective circuits are necessary in an electric bedcover so as to prevent an overheated condition from starting a fire in flammable bedclothes, scorching a fabric portion of the electric bedcover, or injuring a person who may be either asleep or bedridden. An overheated condition may be caused by covering a substantial area of the electric bedcover as by a conventional blanket, by tucking more than unheated marginal portions of the electric bedcover under a mattress, by rumpling a substantial area of the electric bedcover, and otherwise.
2. Description of the Prior Art
It is conventional for an electric bedcover to have a fabric shell, which has two or more plies and a network of serpentine passages within the plies, and an elongated flexible heater, which is deployed through the network of serpentine passages, as exemplified by U.S. Pat. No. 2,203,918 to I. O. Moberg. It also is conventional for a thermostatic device, which is known in its conventional form as an ambient-responsive control, to be used for adjustable control of an on-off cycle of the heater as a function of ambient temperature.
Early electric bedcovers having such ambient-responsive controls are disclosed in U.S. Pat. No. 2,195,958 to W. K. Kearsley and U.S. Pat. No. 2,344,820 to W. K. Kearsley. As disclosed in U.S. Pat. No. 3,708,649 to G. C. Crowley et al., it is common to provide such a bedcover also with an array of safety thermostats which are connected in series between series segments of the heater, and which are located at strategic places within the plies of the fabric shell, and it is known to integrate an ambient-responsive control with the fabric shell.
It is known from U.S. Pat. No. 2,565,478 to G. C. Crowley, U.S. Pat. No. 2,581,212 to D. C. Spooner et al., and U.S. Pat. No. 2,846,560 to Jacoby et al., to employ an elongated flexible component which is deployed as a cable through the network of serpentine passages of the fabric shell of an electric bedcover, which comprises a pair of elongated flexible electrical conductors spaced from each other by a layer of material having a negative temperature coefficient of resistance, and in which one of the conductors constitutes the heater and the other conductor constitutes a carrier for a signal indicative of an overheated condition. An overheated condition causing some part of the layer between the conductors to become a conductor rather than an insulator causes closed contacts of an electromagnetic relay to open so as to deenergize the heater.
It is known from U.S. Pat. No. 2,846,559 to J. Rosenberg, U.S. Pat. No. 3,114,820 to R. G. Holmes, and U.S. Pat. No. 3,222,497 to W. H. Gordon, Jr., to employ an elongated flexible component which acts as a sensor but not as a heater, which is deployed as a cable alongside a separate heater through the network of serpentine passages of the fabric shell of an electric bedcover, and which also comprises a pair of elongated flexible electrical conductors spaced from each other by a layer of material having a negative temperature coefficient of resistance. Each conductor forms a part of a circuit energizing a relay, whose contacts must be closed for the heater to be energized, whereby an overheated condition causing some part of the layer between the conductors to become a conductor rather than an insulator opens the contacts. Cf. U.S. Pat. No. 4,034,185 to G. C. Crowley.
It is known from U.S. Pat. No. 3,418,454 to W. D. Ryckman, Jr., to employ an elongated flexible component which also acts as a sensor but not as a heater, which also is deployed as a cable alongside a separate heater through the network of serpentine passages of the fabric shell of an electric bedcover having an ambient-responsive control as mentioned above, and which also comprises a pair of elongated flexible conductors spaced from each other by a layer of negative temperature coefficient of resistance. Each conductor forms a part of a circuit energizing resistive heaters associated with respective ones of a pair of bimetallic arms of another thermostatic device.
The pair of bimetallic arms of such thermostatic device carry respective contacts, through which the heater is energized, and which are closed when sufficient heat is supplied to each arm by the heater associated with such arm. An overheated condition causing the layer between the conductors of the sensor to become a conductor rather than an insulator divides a current, so as to cause less heat to be supplied by the heater associated with one arm, whereupon such arm tends to open the contacts.
In each circuit wherein an element comprising a pair of conductors separated by a layer of material having a negative temperature coefficient of resistance acts as a sensor but not as a heater as mentioned above, the conductors are intended to carry current during normal operation of the electric blanket. The conductors and associated components of the circuits comprising the conductors have inherent resistance and thus contribute to overall power dissipation of the electric bedcover during its normal operation.
U.S. Pat. No. 2,782,290 to P. E. Lannan et al. discloses, for an electric bedcover, several protective circuits employing an elongated flexible sensor, which comprises a pair of conductors separated by a layer of material having a negative temperature coefficient of rectification. Other protective devices for such electric bedcovers are disclosed in U.S. Pat. No. 3,628,093 to G. C. Crowley, U.S. Pat. No. Re. 28,656 (originally U.S. Pat. No. 3,673,381) to G. C. Crowley et al. and U.S. Pat. No. 3,683,151, No. 3,588,466, and No. 3,588,447, to E. R. Mills et al.
It is known from British Patent Specification No. 1,372,627 to Dimplex Limited, to employ, in an electrical room-heating appliance, a thermistor, by which is understood a compact, rigid, semiconductive component to be mounted on a bracket, but which has a negative temperature coefficient of resistance, in circuits controlling current through a resistor, which is mounted for heat dissipation influencing a thermostatic control for a main heater of the appliance. Similar circuits are understood to have been used in hair dryers.
The application of Richard H. Williams discloses, in an electric blanket having a fabric shell and an ambient-responsive control as mentioned above, a different arrangement wherein an elongated flexible sensor which comprises a pair of conductors spaced from each other by a layer of material having a negative temperature coefficient of resistance, is adapted to influence a bimetallic element of a thermostatic device through a resistor, which is mounted for heat dissipation to the bimetallic element. The sensor and the resistor are connected so as to conduct little current, and thus so as to dissipate little power, except when the layer between the conductor acts as a conductor rather than as an insulator.
In the aforesaid arrangement of said application, the thermostatic device is the ambient-responsive control, to which the resistor is added, and which thus is located outside the fabric shell. As another external lead is required to connect the resistor to the sensor, conventional connectors cannot be used to connect the ambient-responsive control and the circuits confined by the fabric shell.
The aforesaid arrangement of said application may be used advantageously either independently or to supplement conventional safety thermostats. However, if an attempt were made either deliberately or accidentally to power the heater of the electric blanket directly from line voltage, the protective circuit disclosed in said application could be ineffective.
A need remains for a protective circuit which also may be used either independently or to supplement conventional safety thermostats, but which may be confined by the network of serpentine passages within the fabric shell, which does not require any additional leads, which permits standard connectors to be used to connect an ambient-responsive control to the circuits confined by the fabric shell, and which would be effective if the heater of the electric blanket were to be powered directly from line voltage.